LIMITED APPLICATION OF SCHMIDT HAMMER EXPOSURE-AGE DATING IN THE GREAT LAKES REGION

The hardness of a rock exposed at Earth’s surface is related to the amount of time it has been subjected to weathering conditions. Schmidt Hammers quantify rock hardness using rebound values (R-values) from an impact with a calibrated force. The age of rock surfaces and R-values are often inversely related with older, weaker surfaces yielding low R-values and young, stronger surfaces yielding high R-values. Elsewhere, this relationship has been used to construct a Schmidt Hammer exposure-age dating (SHED) calibration curve between R-values and known exposure ages of glacial erratics, often measured with cosmogenic ¹⁰Be dating techniques. When clear relationships between R-values and ¹⁰Be ages are observed, SHED techniques provide a cost-effective and quick means of estimating ages of glacial erratics and general reconstructions of a region’s glacial history. Ages of many glacial landforms across the North American Great Lakes Region are nonexistent, and SHED techniques could provide rapid and widespread age estimation of landforms if a calibration curve can successfully be constructed. Here, we measure R-values on 34 previously dated (with ¹⁰Be) erratics at MIS 3 and LGM moraines in Wisconsin, but preliminary results show significant scatter and appear not to be related. Easily measurable factors that may affect weathering (e.g. erratic size, local climate, elevation, lichen and tree cover, lithology) do not appear to account for this lack of a relationship. Based on these preliminary findings, establishing a SHED calibration curve for the Great Lakes Region does not seem feasible at this time.